Magnetic memory with a domain wall

ABSTRACT

A multi-state low-current-switching magnetic memory element (magnetic memory element) comprising a free layer, two stacks, and a magnetic tunneling junction is disclosed. The stacks and magnetic tunneling junction are disposed upon surfaces of the free layer, with the magnetic tunneling junction located between the stacks. The stacks pin magnetic domains within the free layer, creating a free layer domain wall. A current passed from stack to stack pushes the domain wall, repositioning the domain wall within the free layer. The position of the domain wall relative to the magnetic tunnel junction corresponds to a unique resistance value, and passing current from a stack to the magnetic tunnel junction reads the magnetic memory element&#39;s resistance. Thus, unique memory states may be achieved by moving the domain wall.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/986,802, filed on Jan. 7, 2011, by Rajiv Yadav Ranjan, et al., andentitled “Low Current Switching Magnetic Tunnel Junction Design forMagnetic Memory Using Domain Wall Motion”, which is a continuation ofU.S. patent application Ser. No. 12/255,624, filed on Oct. 21, 2008, byRajiv Yadav Ranjan, and entitled “Low Current Switching Magnetic TunnelJunction Design for Magnetic Memory Using Domain Wall Motion”, whichclaims the benefit of U.S. Provisional Patent Application 61/001,348,entitled “Low Current Switching Magnetic Tunnel Junction Design forMagnetic Memory using Domain Wall Motion,” filed on Oct. 31, 2007, andfrom U.S. patent application Ser. No. 11/674,124, entitled “Non-UniformSwitching Based Non-Volatile Magnetic Base Memory,” filed Feb. 12, 2007,by Ranjan, et al., U.S. patent application Ser. No. 11/678,515, entitled“A High Capacity Low Cost Multi-State Magnetic Memory,” filed Feb. 23,2007, by Ranjan, et al., U.S. patent application Ser. No. 11/776,692,entitled “Non-Volatile Magnetic Memory Element with Graded Layer,” filedJul. 12, 2007, by Ranjan, et al., U.S. patent application Ser. No.11/740,861, entitled “High Capacity Low Cost Multi-Stacked Cross-LineMagnetic Memory,” filed Apr. 26, 2007, by Ranjan, et al., and U.S.Provisional Patent Application No. 61/045,582, entitled “Low PowerNon-Volatile Magnetic Memory Design,” filed Apr. 16, 2008, by Ranjan, etal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel low-current multi-statecurrent-switching magnetic memory, and, more specifically, to a magneticmemory element employing the torque generated by the polarized spins ofelectrons to move a free layer domain wall relative to a magnetic tunneljunction.

2. Description of the Prior Art

Computers conventionally use rotating magnetic media for data storage inthe form of magnetic hard disk drives (HDDs). Though widely used andcommonly accepted, such hard disk drives suffer from a variety ofdeficiencies. Access latency, higher power dissipation, increasedphysical size and inability to withstand any physical shock justifies anew type of storage device. Other dominant semiconductor based storagedevices are DRAM and SRAM which are volatile and very costly, but havefaster random read/write time. Solid-state-nonvolatile-memory (SSNVM)devices like NOR/NAND flash memory provide higher access time, higherIOP speed, lower power dissipation, smaller physical size, and higherreliability, but at a cost which tends to be generally multiple timeshigher than HDDs.

Although NAND flash memory is more costly, it has replaced HDDs in manyapplications such as digital cameras, MP3 players, cell phones, and handheld multimedia devices. However, as process geometry is gettingsmaller, the design of NAND flash memory and DRAM memory is becomingmore difficult to scale. For example, NAND flash memory has issuesrelated to capacitive coupling, few electrons/bit, poor error-rateperformance, and poor reliability due to low read-write endurance. It isbelieved that NAND flash memory, especially multi-bit designs will beextremely difficult to scale below 45 nm-lithography. Likewise, DRAM hasissues related to scaling trench capacitors, necessitating complexdesigns that are becoming difficult to manufacture, and leading tohigher costs.

Currently many platforms use combinations of EEPROM/NOR, NAND, HDDs, andDRAM as a part of the system design. Use of multiple memory technologiesin a single product will add to the design complexity, time to market,and the final cost. For example, a handheld multi-media device whichincorporates NAND Flash, DRAM, and EEPROM/NOR flash will have additionallevels of design complexity, cost more, and take longer to reach themarket than a device incorporating fewer unique memory technologies.Also, incorporating multiple memory technologies increases the device'sfootprint, an undesirable property for a handheld device.

There has also been extensive effort in development of alternativetechnologies such as Ovanic RAM (or phase-change memory), Ferro-electricRAM (FeRAM), Magnetic RAM (MRAM), nanochip, and others, to replacememories used in current designs. Although these various memory/storagetechnologies have created many challenges, there have been greatadvances made in this field in recent years.Magnetic-random-access-memory (MRAM) seems to lead as the best candidateto replace all types of memories as a universal memory solution.Recently, low capacity MRAM, which relies on a magnetic field to switchthe memory elements, started shipping. Numerous scientific papers haveshown that direct electrical current can also be used to switch thememory elements. There is a tradeoff between low-switching current, andreliability of the memory associated due to thermal stability.

The current-switching tradeoff arises due to the interaction betweenmagnetic moments and the electrical transport current. At least twodominant mechanisms have been proposed, namely, (1) current inducedmagnetic field and, (2) spin-torque from the current spin polarization.While the current induced effect through the magnetic field is directlyproportion to radius (r), the later spin-torque effect is proportionalto r², where r is the distance from the center of current carryingdevice. Incidentally, the torque generated by spins through the momentumtransfer of tunneled polarized spins from the fixed layer, which opposesthe “intrinsic” damping of spins of the free-layer. At sufficientcurrent this can reverse the direction of the magnetization in the freelayer. The critical current required for such switching:

$\begin{matrix}{{Ic} = {{{Ic}_{0}\left\lbrack {1 - {\left( \frac{k_{b}T}{K_{u}V} \right){\ln\left( \frac{t_{p}}{t_{0}} \right)}}} \right\rbrack}.}} & {{Equation}\mspace{14mu} 1}\end{matrix}$Where Ic₀ is the critical switching current density without thermalfluctuation; k_(b) is the Boltzman constant; T is the temperature; K_(u)is the effective uniaxial anisotropy; V is the volume of the free-layer;t₀ is the inverse of the procession frequency of the spin (less than 1ns); t_(p) is the pulse width of the switching current.

Equation 1 shows that one way to reduce the critical switching currentdensity is by reducing either Ku or V of the free-layer. Secondly, theswitching current can be reduced by utilizing a thinner free-layer, butthis may compromise reliability by making the memory cell thermallyunstable. A memory element with a free-layer having a higher K_(u)V ismore thermally stable at higher temperatures. A general rule of thumb isthat the magnetic energy, K_(u)V, of the free-layer be greater thanabout 80 k_(b)T where, k_(b) is the Boltzmann constant and T is theambient temperature.

What is needed is a novel memory that has high tunnelingmagneto-resistance (TMR) while having flexibility in the selection ofresistance-area product (RA) and lateral dimensions of the magnetictunnel junction. A high TMR is highly desirable as it enables higheasier sensing between the two states, while flexibility in RA designwith lateral size enables scalability, as well as improving reliability.

SUMMARY OF THE INVENTION

Briefly, a multi-state low-current-switching magnetic memory element(magnetic memory element) using domain wall positioning to define thememory state is disclosed. The magnetic memory element comprises asubstantially flat, elongated free layer, with stacks disposed on eachend. The stacks serve both as electrodes, and, due to each having aunique magnetic moment, pin each end of the free layer in oppositemagnetic directions. The pinned magnetic moments generate a well-defineddomain wall within the free layer. Disposed central to the free layer isa magnetic tunnel junction (MTJ). Current may be passed between thestacks, in either direction, to push the free layer's domain wall in thesame direction as electron current is passed, and relocates the domainwall accordingly. Current may also be passed between a stack and theMTJ, providing a resistance value reading. The resistance value isdependent upon the location of the domain wall relative to the MTJ,therefore allowing the magnetic memory element to “store” variousvalues, dependent upon its magnetic state.

These and other objects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detailed description of the preferred embodiments illustratedin the several figures of the drawing.

IN THE DRAWINGS

FIG. 1 shows the relevant layers of a magnetic tunnel junction of amagnetic memory element.

FIG. 2 shows a three-dimensional perspective view of magnetic memoryelement 200.

FIG. 3 shows a three-dimensional perspective view of the magnetic memoryelement of FIG. 2, being read by electron current 303 passed from theleft electrode 206 to the magnetic tunnel junction 222.

FIG. 4 shows a three-dimensional perspective view of the magnetic memoryelement of FIG. 2, being written by electron current 401 passed from theleft electrode 206 to the right electrode 232.

FIG. 5 shows a three-dimensional perspective view of the magnetic memoryelement of FIG. 2, being read by electron current 501 passed from theleft electrode 206 to the magnetic tunnel junction 222.

FIG. 6 shows a three-dimensional perspective view of the magnetic memoryelement of FIG. 2, being written by electron current 601 passed from theright electrode 232 to the left electrode 206.

FIG. 7 shows a three-dimensional perspective view of the magnetic memoryelement of FIG. 2, capable of multi-level storage by inducing multipledomain wall pinning sites.

FIG. 8 shows a top view of a notched free layer, capable of multi-levelstorage due to multiple domain wall pinning sites.

FIG. 9 shows a three-dimensional perspective view of a magnetic memoryelement arranged with respect to word line and bit line in a memoryarray assembly.

FIG. 10 shows a three-dimensional perspective view of an alternativemagnetic memory element arrangement with respect to the bit lines andword line in a memory array assembly.

FIG. 11 shows the electron flow, through a magnetic memory element arrayassembly, for reading the state of a magnetic memory element.

FIG. 12 shows the electron flow, through a magnetic memory element arrayassembly, for writing a ‘0’ state to a magnetic memory element.

FIG. 13 shows the electron flow, through a magnetic memory element arrayassembly, for writing a ‘1’ state to a magnetic memory element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of the embodiments, reference is made tothe accompanying drawings that form a part hereof, and in which is shownby way of illustration of the specific embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized because structural changes may be madewithout departing from the scope of the present invention. It should benoted that the figures discussed herein are not drawn to scale andthicknesses of lines are not indicative of actual sizes.

In an embodiment of the present invention a magnetic tunnel junction(MTJ) is located central to a free layer, with the free layer havingstacks with pinned magnetic moments on either end, and the end-definingstacks creating a well defined magnetic domain state of the free layertherebetween. Pushing the free layer's domain wall towards either stackmodifies the domain state of the free layer. The domain wall is pushedby the passage of electron current from a stack, laterally through thefree layer, to the other stack. As electron current flows laterallythrough the free layer, the domain wall is pushed in the direction ofthe electron flow due to spin transfer torque that exists between thepolarized electron spin and the magnetic spins in the domain wall whichare not aligned with the electron spins.

Depending on the direction in which electron current is applied, thedomain wall can be pushed towards either stack. As a result, the domainwall can be dynamically positioned along the free layer between thestacks. Located on a lateral face of the connecting free layer region isa MTJ. The domain wall can be positioned within the free layer on eitherside of the MTJ, or anywhere therebetween. In addition to passingcurrent from stack to stack, current can be passed from a stack, throughthe free layer, to the MTJ; or from the MTJ, through the free layer, toa stack. The MTJ's resistance to electron flow is a function of thedomain wall's position within the free layer, and relative to the MTJ.Consequently, the MTJ's resistance to current allows for an accuratemeasurement of the domain wall's position, and thus allows for storageof a bit (by positioning of the domain wall), and reading of that bit(by reading the MTJ's resistance).

A key to the present invention is the creation of a well-defined domainwall by pinning magnetic moments at either end of the free layer. In oneembodiment of the present invention a fixed layer, pinned by ananti-ferromagnetic layer, is located each end of the free layer. One ofthese fixed layers pins the spins of the free layer in a firstdirection, and the other fixed layer pins the spins of the free layer atthe other end in a second, the opposite, direction. This is shown inFIGS. 2-7, and discussed in further detail below.

Referring now to FIG. 1, stack 100 is shown. Stack 100 is comprises ananti-ferromagnetic layer (AFM layer) 106, a fixed layer 104, a tunnelbarrier layer 102, and a free layer 101. Stack 100 is manufactured insuch a manner that fixed layer 104 is formed upon AFM layer 106, tunnelbarrier layer 102 is formed upon fixed layer 104, and free layer 101 isformed upon tunnel barrier layer 102. Free layer 101, tunnel barrierlayer 102, fixed layer 104, and AFM layer 106 are formed directly uponeach other with no intermediate layers, and share the same width anddepth of less than 500 nm, but mainly limited by the lithographytechnology being used to pattern the stack. Free layer 101 has athickness of about 2.5-3 nm; tunnel barrier layer has a thickness ofabout 1.25 nm; fixed layer 104 has a thickness of about 2.5 nm; and AFMlayer 106 has a thickness of about 7 nm.

The composition of AFM layer 106 is typically alloys of IrMn or PtMn.Fixed layer 104 is a compound of Co, Fe, and may contain one or more ofthe elements from the following list: B, P, Ni, Cr, Ta, Zr, Si, Cr, Mo,Hf, and/or W. The composition of tunnel barrier layer 102 is chosenfrom: TiO₂, Al₂O₃, MgO, Ta₂O₅, HfO₂, ZrO₂, TaN, SrO, RuO, and ZnO; andis preferably MgO, or MgO combined with less than 50 molar % of any ofthe previously listed compounds. Free layer 101 is preferably made ofmagnetic materials which have weakly exchange coupled grains; such asone, or more than one, of ferromagnetic elements Co, Fe, and Ni, andcontains less than 20 atomic % of platinum (Pt). Free layer 101 may alsocontain up to 20 atomic % of one or more of: P, B, Cr, Ta, W, Mo, Zr,and/or Hf. AFM layer 106, fixed layer 104, tunnel barrier layer 102, andfree layer 101 are formed using the same processes used to form theanti-ferromagnetic (AFM) layer, fixed layer, tunnel barrier layer, andfree layer, respectively, of U.S. patent application Ser. No.11/674,124, entitled “Non-Uniform Switching Based Non-Volatile MagneticBase Memory,” filed Feb. 12, 2007, by Ranjan, et al.; U.S. patentapplication Ser. No. 11/678,515, entitled “A High Capacity Low CostMulti-State Magnetic Memory,” filed Aug. 23, 2008, by Ranjan, et al.;U.S. patent application Ser. No. 11/776,692, entitled “Non-VolatileMagnetic Memory Element with Graded Layer,” filed Jul. 12, 2007, byRanjan, et al.; U.S. patent application Ser. No. 11/740,861, entitled“High Capacity Low Cost Multi-Stacked Cross-Line Magnetic Memory,” filedApr. 26, 2007, by Ranjan, et al.; and U.S. Provisional PatentApplication No. 61/045,582, entitled “Low Power Non-Volatile MagneticMemory Design,” filed Apr. 16, 2008, by Ranjan, et al.

Fixed layer 104 has a magnetic moment direction pinned into a singledirection through exchange coupling to adjacent AFM layer 106. Themagnetic moment direction of free layer 101 can be changed between theparallel and anti-parallel direction with respect to fixed layer 104.Tunnel barrier layer 102 allows electrons to tunnel between fixed layer104 and free layer 101. When free layer 101's magnetic moment isparallel with fixed layer 104's magnetic moment, the resistance toelectron flow through tunnel barrier layer 102 is lower; and when freelayer 101's magnetic moment is anti-parallel with fixed layer 104'smagnetic moment, the resistance to electron flow through tunnel barrierlayer 102 is higher. This difference in resistance is also known as“tunnel magneto-resistance” or TMR.

The magnetic moments' of the fixed layers of the present invention areset using a magnetic annealing process whereby the magnetic films areexposed to a magnetic field of 4-10 kOe, applied in-plane of the wafersurface, at a temperature usually over 350° C. In one embodiment of thepresent invention, a magnetic field of 10 kOe is applied, in-plane ofthe wafer surface, at 360° C. for 2 hours. In other embodiments of thepresent invention the field exposure time may be shorter, for as shortas a few minutes, or longer. The annealing process causesre-crystallization of the free and fixed layers adjacent to the MgOtunnel barrier layer. This step is critical, as it ensures a high TMR,and impacts the read-speed of the final memory.

Referring now to FIG. 2, magnetic memory element 200 is shown. Magneticmemory element 200 is comprised generally to have a free layer 210,having stacks 206 and 232 positioned on opposite ends of a lateralsurface of free layer 210, and MTJ 222 positioned centrally on a lateralsurface of free layer 210. In FIG. 2, stacks 206 and 232 are shownpositioned on a top surface, while MTJ 222 is positioned on a bottomsurface of magnetic memory element 200. Stacks 206, 232, MTJ 222, andfree layer 210 all have substantially the same depth, less than 500 nm,but mainly limited by the lithography technology being used to patternthe stack.

In an alternative embodiment of the present invention, one or bothstacks are positioned on the same free layer 210 surface as MTJ 222 is.This would be the bottom surface of free layer 210 in FIGS. 2-7. Thus,stacks 206 and/or 232 could extend from the same surface of free layer210, or from opposite surfaces of each other. For example, stack 206might extend from the top surface of free layer 210 while MTJ 222 andstack 232 extend from the bottom surface of free layer 210; or bothstacks 206 and 232 extend from the bottom surface with MTJ 222. Thereference to a “top surface” or a “bottom surface” of free layer 210 isused strictly in reference to drawings, to assist in orientation, and“surfaces” refers to the flat lateral sides of free layer 210 with thegreatest surface area. The top surface is so called because it is closerto the top of the page as viewed in FIGS. 2-7, while the bottom surfaceof free layer 210, hidden underneath and not visible due to theperspective view, is closer to the bottom of the page.

Stack 206 comprises AFM layer 202 formed upon fixed layer 204, and fixedlayer 204 is formed upon free layer 210. The layers of stack 206 havesubstantially the same depth and width, forming a generally uniformraised structure. Stack 232 comprises AFM layer 230 formed upon fixedlayer 228, which is formed upon spacer layer 226, and spacer layer 226is formed upon free layer 210. The layers of stack 232 havesubstantially the same depth and width, forming a generally uniformraised structure.

AFM layers 202 and 230 pin the directions of the magnetic moments offixed layers 204 and 228, respectively. AFM layers 202 and 230 areformed by the same process, and share the same properties, as AFM layer106. Fixed layers 204 and 228 are formed by the same process, and sharethe same properties, as fixed layer 104. Spacer layer 226 comprises athin layer of Ru, typically 0.8-1 nm thick. Spacer layer 226 reversesthe effective magnetic exchange coupling between fixed layer 228 to freelayer 210. This is necessary because the magnetic annealing of a waferis a global process affecting the whole wafer, aligning the magneticmoments of all of the AFM layers (202, 230, and 220), and consequentlypinning the magnetic moments of fixed layers adjacent to the AFMlayers—fixed layers 204, 226, and 218, respectively—in the samedirection. In order to create a well-defined domain wall 235 within freelayer 210, the magnetic moments of stacks at each end of free layer 210(stacks 206 and 232) should have magnetic moments anti-parallel to eachother.

Free layer 210 is shown to have two domains, anti-parallel domain 208and parallel domain 224, anti-parallel domain 208 and parallel domain224 being separated by domain wall 235. Domain wall 235 is an interfaceor transition between the parallel magnetic field of parallel domain 224and the anti-parallel magnetic field of anti-parallel domain 208.“Parallel” and “anti-parallel” are used in reference to MTJ 222. Asshown in FIGS. 2-7, the magnetic moment of parallel domain 224 isdesignated by arrows pointing to the right, in the same direction as themagnetic moment of fixed layer 214 of MTJ 222. Anti-parallel domain208's magnetic moment is designated by arrows pointing to the left,which is anti-parallel to the magnetic moment of fixed layer 214 of MTJ222. Domain wall 235 spans a distance of about 100-150 atoms.

In the following disclosures and figures, the free layers (e.g. freelayer 210) are referred to and depicted as being rectangular, with fourflat lateral sides and 2 flat ends. Further, two of the lateral sidesare depicted as having significantly greater surface area than the othertwo lateral sides, the two lateral sides with the greater surface areabeing referred to as the “top surface and “bottom” surface of the freelayers. This thin, flat, but deep conformation lends itself to asimplified manufacturing process, reduces the cross-sectional area, andbetter defines the free layer's domain wall, but other conformations areconsidered. Other conformations may include a free layer having fourlateral sides of all the same size (square extruded), or triangularextruded, or any other geometric shape where a well-defined domain wallcan be maintained within.

MTJ 222 is formed on a lateral surface of free layer 210, substantiallycentral to the surface, and may be on a lateral side opposite of stacks206 and 232, and. MTJ 222 comprises a tunnel barrier layer 212, fixedlayers 214 and 218, spacer layer 216, and AFM layer 220. MTJ 222 isformed upon the lateral surface of free layer 210, upon which is formedfixed layer 214, upon which is formed spacer layer 216, upon which isformed fixed layer 218, and upon which is formed AFM layer 220. The twofixed layers 214 and 218 have opposite magnetic moments (also known as a“synthetic anti-ferromagnet) which reduces the magneto-staticinteraction between the fixed layers and free layer 210. The MTJ 222have substantially the same depth and width, forming a generally uniformraised structure off of a free layer 210 lateral surface.

Free layer 210 is made of weakly exchange coupled magnetic materials.This allows for the magnetic domains 208 and 224 to be more easilyshrunken and enlarged. Free layer 210 carries the spin transfer currentfrom stack-to-stack, or stack-to-MTJ. The cross sectional area of freelayer 210 can be made very small, especially compared to the lateraldimensions of the free layer 210 or the MTJ 222, making domain wall 235amenable to movement by the passage of a low current.

Tunnel barrier layer 212 is a non-conducting crystalline layer thatallows the passage of electrons from free layer 210 to fixed layer 214,or in the opposite direction, from fixed layer 214 to free layer 210through tunneling. Tunnel barrier layer 212 is identical to tunnelbarrier layer 102 in function, composition, and thickness, as describedin detail above.

In FIG. 2, as indicated by the arrows in free layer 210, domain wall 235is closer to stack 206 (located within free layer 210 between stack 206and MTJ 222), and thus parallel domain 224 of free layer 210 is adjacentto tunnel barrier layer 212. As explained above parallel domain 224'smagnetic moment is anti-parallel to the magnetic moments of fixed layers204 and 228, but is parallel to that of fixed layer 214. When paralleldomain 224 of free layer 210 is located adjacent to tunnel barrier layer212 the resistance experienced by any current passing through to, orfrom, MTJ 222 is less than the resistance experienced when domain wall235 is pushed to the other side of MTJ 222, and anti-parallel domain 208is located adjacent to tunnel barrier layer 212.

The conducting spacer layer 216 is identical to spacer layer 226 infunction, and possible composition and thickness, as described above indetail. Spacer layer 216 is located in between fixed layers 218 and 214and inverts the magnetic exchange coupling between fixed layers 218 and214. Consequently, the magnetic moment of fixed layer 214 isanti-parallel to the magnetic moments of all other fixed layers (204,228, 218) in magnetic memory element 200.

Referring now to FIG. 3, the memory state of magnetic memory element 200is being read. The magnetic state of memory element 200 is read bypassing either current 303 or current 301 through magnetic memoryelement 200. Current 303 is applied at AFM layer 202 of stack 206,passes through stack 206, into and laterally through free layer 210, andthen out MTJ 222 through AFM layer 220. Current 301 is applied at AFMlayer 220 of MTJ 222, passes through MTJ 222, into and through freelayer 210, and then out of stack 206 through AFM layer 202. When domainwall 235 is positioned such that parallel domain 224 is adjacent totunnel barrier layer 212, the electron spin orientations in free layer210 and fixed layer 214 are parallel, and currents 301 and/or 303experience a low resistance state. Alternatively, the magnetic state ofmemory element 200 can be read by passing the current through stack 232instead of stack 206.

In FIG. 4, the memory state of magnetic memory element 200 is beingwritten. Electron current 401 is applied at AFM layer 202 of stack 206,passes through stack 206, into and laterally through free layer 210, andthen out of stack 232 through AFM layer 230. As electron current 401passed through free layer 401, it pushes domain wall 404 to the right,towards stack 232. Domain wall 404, as shown, is one possible positionof the domain wall within free layer 210 as it is pushed from stack 206,towards stack 232, and is a repositioned domain wall 235.

Referring now to FIG. 5, the memory state of magnetic memory element 200is again being read. In FIG. 5, current passed from stack 206 to stack232 (shown in FIG. 4 as current 401) has moved domain wall 504 to theother side of MTJ 222. In this state, domain wall 504 is between MTJ 222and stack 232, and anti-parallel domain 208 of free layer 210 is nowadjacent to, and interfaced with, tunnel barrier layer 212. To read thememory state of magnetic memory element 200, either current 501 orcurrent 503 is passed through magnetic memory element 200. Current 501is applied to AFM layer 202 of stack 206, passes through stack 206, intoand laterally through free layer 210, and then through MTJ 222, outthrough AFM layer 220. Current 503 is applied to AFM layer 220 of MTJ222, passes through MTJ 222, into and laterally through free layer 210,and then through stack 206, out through AFM layer 202. Alternatively,the magnetic state of memory element 200 can be read by passing thecurrent through stack 232 instead of stack 206.

When domain wall 504 is positioned so that anti-parallel domain 208 isadjacent to tunnel barrier layer 212, the magnetic spin orientations ofadjacent free layer 210 and neighboring fixed layer 214 areanti-parallel, and currents 501 and/or 503 experience a high resistancestate. This high resistance state, experienced by currents 501 and 503when passing from anti-parallel domain 208 through tunnel barrier layer212 to fixed layer 214, is higher than the resistance experienced bycurrents 301 and 303 when passing through parallel domain 224, tunnelbarrier layer 212, and fixed layer 214. Because these differentresistance states are a direct function of the positioning of the domainwall (domain walls 235 and 504 within FIGS. 2-5), the domain wallpositioning effectively represents either a ‘0’ or a ‘1’ state ofmagnetic memory element 200.

In FIG. 6, the memory state of magnetic memory element 200 is againbeing written. Electron current 601 is applied at AFM layer 230 of stack232, passes through stack 232, into and laterally through free layer210, and then out of stack 206 through AFM layer 202. Electron current601 pushes domain wall 604 to the left, towards stack 206. Domain wall604, as shown, is one possible position of the domain wall within freelayer 210 as it is pushed from stack 232, towards stack 206. Eventually,domain wall 604 will be returned to domain wall 235 in the positionshown in FIG. 2, and consequently magnetic memory element 200 will bereturned to the associated state.

The currents 301, 303, 501 and 503 typically have a much smallermagnitude than the current 401 and 601. The magnitude of currents 301,303, 501, and 503 is reduced in order to prevent domain wall motionduring the read out process.

Referring now to FIG. 7, in alternative embodiments of the presentinvention, memory element 200 may be capable of storing more than 2memory states, i.e., more than just ‘0’ or ‘1’. This is possible byusing electron currents 401 and 601 to relocate the domain wall of freelayer 410 to more than two positions. In these alternative embodiments,the domain wall within free layer 410 may be positioned where domainwalls 404 and 604 are depicted positioned previously, as well as otherpositions. In FIGS. 4 and 6, respectively, domain walls 404 and 604 arepositioned such that tunnel barrier layer is adjacent to areas ofanti-parallel domain 208 and parallel domain 224 concurrently. Thissub-division of memory states is possible by applying an electron pulse(current) that pushes the domain wall only a limited distance. When thedomain wall is in these intermediate positions, resistance valuesbetween the previously described high resistance and low resistancestates are obtained.

As shown in FIG. 7, domain wall 404 is positioned in a manner that makesa larger area of parallel domain 224 adjacent/interfaced to tunnelbarrier layer 212 than anti-parallel domain 208. Domain wall 704 ispositioned in a manner that makes equal areas of anti-parallel domain208 and parallel domain 224 adjacent/interfaced to tunnel barrier layer212. Domain wall 604 is positioned in a manner that makes a larger areaof anti-parallel domain 208 adjacent/interfaced to tunnel barrier layer212 than parallel domain 224.

The resistance of MTJ 222 is a function of the surface areas of paralleldomain 224 and of anti-parallel domain 208 that are interfaced withtunnel barrier layer 212. The resistance to current decreases as tunnelbarrier layer 212 is interfaced with a greater surface area of paralleldomain 224, and lesser area of anti-parallel domain 208. And,conversely, the resistance to current increases as tunnel barrier layer212 is interfaced with a greater surface area of anti-parallel domain208, and lesser area of parallel domain 224. Thus, a read operation ofmagnetic memory element 200 with domain wall 404 would experience lessresistance than a read operation of magnetic memory element 200 withdomain wall 704. A read operation of magnetic memory element 200 withdomain wall 604 would have a greater resistance than domain walls 404 or704. All of these resistance values would be between the high resistancestate (domain wall 235) and low resistance state (domain wall 504),shown in FIGS. 2 and 5 respectively.

Referring to FIG. 8, another method of achieving multiple levels ofmemory states is shown, in an alternative embodiment of the presentinvention. In FIG. 8, a top view of free layer 810 of magnetic memoryelement 800 is shown. Central to the bottom surface of free layer 810,hidden below notch region 811 and not visible in FIG. 8, is an MTJ, MTJ818, comprised of the same layers as MTJ 222 of magnetic memory element200. The notches 812, 814, and 816 in free layer 810 all correspond to apreferred domain wall position. At notches 812, 814, and 816, thecross-sectional area of free layer 810 is at a minimum, and thus thedomain wall thickness is reduced, and better defined. As the domain wallis pushed to the narrowest part of each notch, it accelerates, due tothe decreasing cross-sectional surface area; and as the domain wall ispushed out of a notch, it does so slowly, due to the increasingcross-sectional surface area. As with memory element 200, each notchposition is associated with a different resistance, with resistanceincreasing as a larger region of anti-parallel magnetic domaininterfaces with the tunnel barrier layer of MTJ 818. This creates aneffective way to “lock” the domain wall in consistent positions, andthus makes multi-level storage more reliable.

Referring now to FIG. 9, a three-dimensional perspective view of amagnetic memory array 900 is shown in one embodiment of the presentinvention. Magnetic memory array includes magnetic memory element 200,with MTJ 222, free layer 210, and stacks 232 and 206 labeledappropriately. On AFM layer 230 of MTJ 222, bit line read 902 iscoupled; on AFM layer 230 of stack 232, bit line write 903 is coupled;and on AFM layer 202 of stack 206, pinning layer 905 is coupled, whichhas word line read/write formed thereupon.

Magnetic memory element 200 is inverted, with respect to how it is shownin FIGS. 2-7, so that MTJ 222 extends upward from free layer 210, andstacks 232 and 206 extend downward from free layer 210. Consequently,MTJ 222 is the top-most part of magnetic memory element 200, and bitline read 902, above MTJ 222, is the highest line. Bit line read 902 isparallel to bit line write 903. Bit line write 903 runs below memoryelement 200, perpendicular to the lengthwise direction of free layer210. Pinning layer 906 is formed off of AFM layer 230, and pinning layer906 extends downward, coupled to word line read/write 904. The pinninglayer 906 has the same function as pinning layer 204 in FIGS. 2-7. Wordline read/write 904 runs parallel to free layer 210, below bit linewrite 903, and at a distance of the height of pinning layer 905 plus theheight of stack 206. An important feature of array 900 is that bit lineread 902 and bit line write 903 are in different planes. Bit line read902 is connected to MTJ 222, as current is only passed through it toread the memory state of the memory element; bit line write 903 isconnected to stack 232, as current passes through it only to write thememory state of the memory element; and word line read/write 904 isconnected to stack 206, as current passes through it for both readingand writing the memory state of the memory element.

In FIG. 9, to simplify the drawing, only one magnetic memory element isincluded in array 900, but in practice many magnetic memory elements maybe attached to word line read/write 904, bit line write 903, and bitline read 902. This direct connection can be modified to include aselect transistor. In such a configuration, the word line is connectedto the gate of the transistor, the left side fixed layer is connected toone end of the transistor, and the other end of the transistor can be atlow (e.g. ground) or high potential, depending on the direction of theread current and the direction of the write current.

Referring to FIG. 10, a three-dimensional perspective view of magneticmemory array 1000 is shown in an alternative embodiment of the presentinvention. In this array, the bit line arrangement and the memoryelement are configured differently than with memory element 200 or array900. In array 1000, bit line read 1002 and bit line write 1003 areparallel to each other and in the same plane. The magnetic memoryelement of array 1000 is configured in a manner so that MTJ 1022 andstack 1032 extend off of the same (top) surface of free layer 1010, andstack 1006 extends off of the other (bottom) surface of free layer 1010.Stack 1006 is structurally and functionally identical to stack 206, freelayer 1010 is structurally and functionally identical to free layer 210,MTJ 1022 is structurally and functionally identical to MTJ 222, andstack 1032 is structurally and functionally identical to stack 232. Therelocation of one of the stacks, stack 1032, to the same surface of freelayer 1010 as the MTJ, MTJ 1022, does not cause the magnetic memoryelement to operate any differently than as discussed above for memoryelement 200.

As oriented, bit line read 1002 and bit line write 1003 are the top mostcomponents, with word line read/write 1004 the bottom most component,and the magnetic memory element sandwiched between. This alternativeorientation provides an alternative method of laying out the magneticmemory circuitry.

Referring now to FIGS. 11-13, reading and writing current application tothe magnetic memory arrays 900 or 1000 is shown. In FIGS. 11-13, array1100 is shown consisting of 25 (twenty-five) magnetic memory elements1101, each having two stacks and an MTJ, and all connected to a wordline read/write, bit line read, and bit line write. Of interest ismagnetic memory element 1108, connected to word line read/write 1104 atone stack, bit line write 1103 at a second stack, and bit line read 1102at its MTJ.

In FIG. 11, current 1111 is applied to word line read/write 1104, whichtravels through magnetic memory element 1108, and out through bit lineread 1102. Because word line read/write 1104 is connected to magneticmemory element 1108 at a stack, e.g. stack 206 or 1006, and bit lineread 1102 is connected to magnetic memory element 1108's MTJ, e.g., MTJ222 or 1022, application of current 1111 will provide a resistance valuefor reading the memory state of magnetic memory element 1108.

In FIG. 12, current 1211 is applied to word line read/write 1104, whichtravels through magnetic memory element 1108, and out through bit linewrite 1103. Because word line read/write 1104 is connected to magneticmemory element 1108 at one stack, e.g. stack 206 or 1006, and bit linewrite 1103 is connected at magnetic memory element 1108's second stack,e.g. stack 232 or 1032, application of 1211 pushes the domain wallwithin magnetic memory element 1108's free layer towards the secondstack connected to bit line write 1103, and writes a ‘0’ value to themagnetic memory element.

In FIG. 13, current 1311 is applied to bit line write 1103, whichtravels through magnetic memory element 1108, and out through word lineread/write 1104. Current 1311 pushes the domain wall of magnetic memoryelement 1108 in a direction opposite to how it's pushed by current 1211,towards the first stack connected to word line read/write 1104, andconsequently a ‘1’ value is written to magnetic memory element 1108.

As known by one of ordinary skill in the art, the present invention,including any logic circuit or transistor circuit, may be modeled,generated, or both by computer based on a description of the hardwareexpressed in the syntax and the semantics of a hardware descriptionlanguage (HDL). Such HDL descriptions are often stored on a computerreadable medium. Applicable HDLs include those at the layout, circuitnetlist, and/or schematic capture levels. Examples of HDLs include, butare not limited to: GDS II and OASIS (layout level); various SPICElanguages, and IBIS (circuit netlist level); and Virtuoso custom designlanguage and Design Architecture-IC custom design language (schematiccapture level). HDL descriptions may also be used for a variety ofpurposes, including but not limited to layout, behavior, logic andcircuit design verification, modeling, and/or simulation.

Although the present invention has been described in terms of specificembodiment, it is anticipated that alterations and modifications thereofwill no doubt become apparent to those more skilled in the art. Suchalterations and modifications include, for example, extending the stacksand magnetic tunnel junction from free layer in variousthree-dimensional conformations, normal to the substrate surface orstacked planes on top, in order to maintain higher capacity. It istherefore intended that the following claims be interpreted as coveringall such alterations and modification as fall within the true spirit andscope of the invention.

What is claimed is:
 1. A magnetic memory element capable of storing aplurality of magnetic states, each state representative of a logicvalue, the magnetic memory element comprising: a free layer having atleast one lateral surface on top of which are formed at least twostacks, the free layer having a domain wall, the domain wall having analterable position, the position of the domain wall defining the stateof the magnetic memory element; a first stack disposed upon the lateralsurface of the free layer; a second stack disposed upon the lateralsurface of the free layer; and a magnetic tunneling junction (MTJ)disposed upon the free layer, between the first stack and the secondstack, wherein the position of the domain wall relative to the MTJdefines the magnetic state of the magnetic memory element, furtherwherein the number of magnetic states is two.
 2. A magnetic memoryelement, as recited in claim 1, wherein the number of magnetic states ismore than two.
 3. A magnetic memory element, as recited in claim 1,wherein the magnetic memory element has a resistance value and eachposition of the domain wall is indicative of a unique resistance value.4. A magnetic memory element, as recited in claim 1, wherein themagnetic moment of the free layer is in plane relative to the surface onwhich the magnetic memory element is formed.
 5. A magnetic memoryelement, as recited in claim 1, wherein the magnetic moment of the freelayer is perpendicular relative to the surface on which the magneticmemory element is formed.
 6. A magnetic memory element, as recited inclaim 1, wherein the magnetic memory element has associated therewith aresistance value and each position of the domain wall corresponds to aunique resistance value, further wherein the MTJ has a tunnel barrierlayer formed adjacent to the free layer and the position of the domainwall defining the region of the anti-parallel domain and the paralleldomain, wherein the resistance value increasing as a larger region ofanti-parallel domain interfaces with the tunnel barrier layer therebylocking the domain wall in consistent positions.
 7. A magnetic memoryelement, as recited in claim 1, wherein the first stack includes a firststack spacer layer formed on the lateral surface of the free layer, afirst stack fixed layer formed on top of the first stack spacer layerand an first stack anti-ferromagnetic (AFM) layer formed on top of thefirst stack fixed layer.
 8. A magnetic memory element, as recited inclaim 7, wherein the first stack spacer layer is operative to reverse aneffective magnetic exchange coupling between the first stack fixed layerand the free layer.
 9. A magnetic memory element, as recited in claim 7,wherein the first stack spacer layer is made of Ru.
 10. A magneticmemory element, as recited in claim 9, wherein the second stack is madeof a second stack fixed layer formed on top of the lateral surface ofthe free layer and a second stack AFM layer formed on top of the secondstack fixed layer.
 11. A magnetic memory element, as recited in claim 1,wherein the first stack and second stack AFM layers are each made ofIrMn or PtMn.
 12. A magnetic memory element, as recited in claim 1,wherein MTJ is made of a MTJ AFM layer, a MTJ fixed layer formed on topof the MTJ AFM, a MTJ spacer layer formed on top of the MTJ fixed layerand a second MTJ fixed layer formed on top of the MTJ spacer layer and atunnel barrier layer formed on top of the second MTJ fixed layer.
 13. Amagnetic memory element, as recited in claim 12, wherein the MTJ fixedlayer and the second MTJ fixed layer are made of a compound of Co and Feand further are made of a material selected from a group consisting of:B, P, Ni, Cr, Ta, Zr, Si, Cr, Mo, Hf, and W.
 14. A magnetic memoryelement, as recited in claim 1, wherein the free layer is made ofmagnetic materials which have weakly exchange coupled grains.
 15. Amagnetic memory element, as recited in claim 1, wherein the free layeris made of one or more of materials selected from a group consisting of:Co, Fe and Ni, and comprises less than 20 atomic percentage of Pt.
 16. Amagnetic memory element, as recited in claim 1, wherein the free layeris made of up to 20 atomic % of one or more of: P, B, Cr, Ta, W, Mo, Zr,and/or Hf.
 17. A magnetic memory element, as recited in claim 1, whereincurrent is applied at the first stack to the magnetic memory elementduring a read operation.
 18. A magnetic memory element, as recited inclaim 1, wherein current is applied at the second stack to the magneticmemory element during a read operation.
 19. A magnetic memory element,as recited in claim 1, wherein the MTJ is made on a bottom surface ofthe free layer that is opposite the lateral surface thereof.
 20. Amagnetic memory element, as recited in claim 12, wherein the MTJ is madeon the lateral surface of the free layer.
 21. A magnetic memory element,as recited in claim 1, wherein each of the first and second stacks is aSTTMRAM.